1. Field of Invention
The invention relates generally to protective devices for detecting overcurrent conditions in an alternating electric power circuit, and, more particularly, to a circuit for raising the minimum overcurrent value sensed by these protective devices for a limited time each time the power system is re-energized after having been opened.
2. Prior Art
Each time an electric power circuit is energized there is a transient inrush current that rapidly rises to a peak and then decays to a normal load current level in a period of time which may range from a fraction of a second to several minutes, depending on the type and diversity of loads on the power circuit and the length of time the circuit has been de-energized.
These transient inrush currents often cause application problems with both enclosing circuit breakers and sectionalizer switches. For example, on sectionalizers feeding different loads from a common line which has a fault interrupter, a permanent fault on one branch line causes the other branch sectionalizers to arm due to the inrush current, then count or trip on subsequent operations of the fault interrupter.
Reclosers usually have two fast fault openings to clear branch line faults before the tap fuses can blow, followed by two slow fault openings to allow the tap fuses to blow. In some cases, the inrush current to a power circuit exceeds the fast time-current tripping characteristic of the recloser for that circuit, thus producing unnecessary operations of this recloser. Also, at times, the minimum overcurrent tripping value of a recloser must be set at a relatively low value in order to coordinate with upline backup devices, and in such a case, the inrush current can cause such a recloser to operate to lockout.
Typically, the overcurrent pickup circuits of both reclosers and sectionalizer switches are activated by the voltage drop across a resistive element caused by the flow of a DC current through this resistive element which is proportional to the phase or ground currents of the electric power circuit. In known inrush current restraint circuits for sectionalizer switches, such as those disclosed in U.S. Pat. No. 3,571,661, issued Mar. 23, 1971 to Blaine H. Schultz, and U.S. Pat. No. 3,739,229, issued June 12, 1973 to myself, when voltage or current is initially sensed at the sectionalizing switch following a period of de-energization, a relay is energized for a selected period of time to connect a second resistive element in parallel with the resistive element connected across the input to the overcurrent pickup circuit, and thus bypass a portion of the current which is proportional to the phase or ground current. In this way, the minimum overcurrent pickup value is increased to a higher constant value for a constant period of time following re-energization of the power line.
In both of these known inrush current restraint circuits, the overcurrent pickup value is increased by a fixed percentage above the normal pickup value, whereas an actual inrush current to a power circuit generally rises relatively rapidly to a peak value, then decays at a slower rate back to its normal value. Ideally, an inrush current restraint circuit, which modifies the overcurrent pickup value to closely follow the inrush current characteristics of the power system would still provide the minimum necessary restraint, and, at the same time, provide the most sensitive protection against low level faults.